Radio frequency amplifiers designed for modern wireless communication system formats such as Code Division Multiple Access (CDMA), Universal Mobile Telecommunications System (UMTS), Orthogonal Frequency-division Multiple Access (OFDMA), Long Term Evolution (LTE), and multi-carrier systems that have significant signal peak-to-average requirements, need to balance conflicting performance characteristics such as linearity, efficiency, high signal output power, and wide operational bandwidth. That is, the characteristics of good linearity, high efficiency, and high power operation over a wide bandwidth, while desired, cannot easily be achieved simultaneously in the same amplifier. For example, Class A amplifiers can be very linear and have wideband performance but are inefficient at all output power levels and may be completely impractical at high power levels for reasons of power consumption and heat generation. Class AB amplifiers have decent linearity and high power operation with moderate bandwidths but poor efficiency, whereas amplifiers operating as class C or D have poor linearity but high efficiency. Moreover, most high efficiency techniques such as Class E and F can operate only over limited bandwidths or limited instantaneous bandwidths such as with the current state of the art for various Envelope Modulation amplifiers.
In an attempt to optimally combine linearity and efficiency, a class of high power radio frequency (RF) power amplifiers known as Doherty amplifiers are often employed. Doherty amplifiers combine good linearity and efficiency by operating an amplifier of a primary, or carrier, amplifier stage as, for example, class AB, in parallel with an auxiliary, or peaking, amplifier stage having an amplifier operating as, for example, class C. A signal splitter coupled to an input of each stage splits an input signal to drive the two amplifiers, and a signal combiner coupled to an output of each stage combines the two output signals. When the input signal level is low, the amplifier operates efficiently because the auxiliary amplifier stage is completely cutoff and consumes no power while the primary stage is run into an efficiency enhancing power saturating load. When the input signal level is high, the peaking amplifier turns on through self rectification from the input signal amplitude, the loading on the output of the primary amplifier transitions to that of the system characteristic impedance, and both amplifier stages may deliver up to their maximum power levels and peak efficiencies.
Prior art Doherty amplifiers provide significantly improved efficiency at power levels or in power “back-off” modes that are sufficiently less than the peak output capability of the combined amplifier devices, with sufficient bandwidth to cover most commercial bands of interest—but not typically multiple bands of commercial interest. For example, if fractional bandwidth is defined as Fbw=100×2(f2−f1)/(f1+f2), then the LTE Band XIII 743-734 MHz has a fractional bandwidth of 2.4%, the U.S. Cell Band at 839-894 MHz has 2.8%, and the 900 MHz GSM at 925-930 MHz has 3.7%. The best prior art might be able to cover both the 839-894 MHz and 925-930 MHz bands with a single amplifier for a fractional bandwidth of about 10% but not all 3 bands above with an effective 25% fractional bandwidth. Included among the many reasons for bandwidth limitation found in most of the prior art are:
(1) A reliance on fixed frequency limiting quarter-wave transmission structures such as transmission line transformers or hybrid coupler transformers for Doherty load modulation, off-state impedance optimization of the peaking amplifier output loading at the Doherty power combine point, power combine phasing, and impedance transformation from the Doherty combine point to the system impedance.
(2) Adherence to the classic Doherty non-inverting topology with its higher than the system impedance saturating load presented to the output of the carrier amplifier match rather than the inverted Doherty topology which presents a lower than system impedance saturating load to the output of the carrier amplifier output match during cutoff of the peaking amplifier and has a broader bandwidth when implemented with a resonant low-Q multiple low-pass matching section solution.
(3) Use of a larger number of hybrid combiner based phase shifting elements in the output the classic Non-inverting Doherty than required, that is, more than two which introduces both additional output power loss and some reduction in the bandwidth.
(4) Failure to use power amplifying devices with minimal output capacitance so that bandwidth limiting impedance expansion over frequency during load modulation of the carrier amplifier can be minimized.
For these reasons, high power Doherty amplifiers of the prior art are tailored mostly to single band operation only and are not suitable for multi-band operation.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. Those skilled in the art will further recognize that references to specific implementation embodiments such as “circuitry” may equally be accomplished via replacement with software instruction executions either on general purpose computing apparatus (e.g., CPU) or specialized processing apparatus (e.g., DSP). It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.